If your paper is less than 300gsm I would definately stretch it. I put the paper in a few inches of water in the bath for 30 secs, then lay it on a wooden board, taping it down each edge with 2cm gum tape. I dry this flat, making sure it doesn't dry too quickly and burst from the tape. If this looks as though it's going to happen, I use extra tape (often parcel tape). If it's a big sheet of paper over 300gsm, it's also worth considering stretching it.